1. Field of the Invention
This invention relates to switching converters, and more particularly to reducing ripple in a differential output switching converter.
2. Prior Art
A typical synchronous buck converter circuit includes a pair of metal oxide semiconductor field-effect transistors (MOSFET), an output filter, and a controller that provides the synchronous switching function. FIG. 1 illustrates the simplified schematic diagram of a synchronous buck converter 100. In the buck converter 100, the controller and gate driver 102 controls the output voltage 104. If the output voltage 104 falls below the lower limit of the regulation level, the controller/driver 102 turns on MOSFET Q1 and turns off MOSFET Q2. Thus, the controller/driver 102 transitions Q1 and Q2 into a xe2x80x9cswitch ONxe2x80x9d state, where the full input voltage (VDD) is applied to the LC filter of inductor L1 and capacitor C2. Substantially simultaneously, the current feeds the output load, RLOAD. When the output voltage 104 exceeds the upper limit of the regulation level, the controller/driver 102 turns off MOSFET Q1 and turns on MOSFET Q2. This is the xe2x80x9cswitch OFFxe2x80x9d state, where the input voltage is zero. Transistor Q2 provides an alternate path for the stored energy in inductor L1 to be delivered into the load, RLOAD. Furthermore, the path has no sense of direction of current so that negative current is allowed.
The controller/driver 102 ensures that power MOSFETs Q1 and Q2 are not turned on simultaneously. Turning on both Q1 and Q2 simultaneously would place a momentary short across the input power bus resulting in much lower efficiencies and potential destruction of the switching devices.
FIG. 2 shows a simplified representation of switching control. The controller/driver 102 may turn on MOSFET Q1 and turn off Q2 according to a programmed duty cycle. This causes the output voltage 104 to increase, as shown, within the xe2x80x9cswitch ONxe2x80x9d state 202. The programmed duty cycle within the controller/driver 102 may then turn off MOSFET Q1 and turn on Q2. This causes the output voltage 104 to decrease, as shown, within the xe2x80x9cswitch OFFxe2x80x9d state 204. Therefore, the switching control keeps the output voltage 104 within the band 200 around the reference voltage.
Two synchronous buck converters may be configured to provide bi-directional voltage and/or current from a single polarity power source. To generate this bi-directional voltage, the output load may be connected between the filtered outputs of the two converters. Moreover, the voltage across the load may be adjusted by increasing the duty cycle of the first converter while decreasing the duty cycle of the second converter. However, this technique is disadvantageous when trying to get 0 volt across the load because both converters must operate with 50% duty cycle. In this case, the output voltage ripple of both converters may add to create an even larger output ripple.
In one aspect, the present disclosure describes a differential switching converter system. The converter system includes a plurality of switching converters having at least first and second converters. The plurality of switching converters is coupled in a differential configuration across an output load. Each converter has at least first and second transistors configured to apply and receive energy to/from an energy storage element. In one embodiment, the energy storage element includes an inductor. Each converter is configured to regulate energy transfer from input to output while maintaining a constant output level within load limits.
The converter system also includes a controller, which is configured to control the operation of the first and second transistors of each converter. The controller synchronizes turn-on and turn-off of the first transistor in the first converter with that of the first transistor in the second converter. The controller also controls turn-on and turn-off of the second transistors in the first and second converters to provide first and second duty cycles, respectively. Therefore, the controller operates to substantially reduce ripple in the constant output level.
In another aspect, the present disclosure describes a method for substantially reducing ripple on a load. The method includes turning on high side transistors of switching converters, substantially simultaneously. The high side transistors are then turned off, and the low side transistors are turned on, in response to a programmed duty cycle for each of the switching converters.